Implementing JavaScript Promise in 70 lines of code!

Ever wondered how JavaScript Promise works internally? Obviously there is some native browser support involved, right? Nope! We can implement the Promise interface using pure JavaScript, examples are libraries like Bluebird or Q. And it’s much simpler than you may think, we can do so in only 70 lines of code! This will help with gaining a deeper insight into Promises by demystifying the underlying formation. Can also serve as a good interview question, if you are an evil employer (don’t be!). Let’s dig into it!

First thing that you notice is that a Promise has three states, so should we:

const states = {
    pending: 'Pending',
    resolved: 'Resolved',
    rejected: 'Rejected'
};

Using a class sounds reasonable since we should be able to create a

new Promise()

. Ah, and let’s name our class something else! It’s an object that can

resolve

reject

. Hmm, google thinks that

Nancy

is capable of those! Let’s go with that:

class Nancy {
    constructor(executor) {
        const resolve = () => {
            this.state = states.resolved;
        };

        const reject = () => {
            this.state = states.rejected;
        };

        this.state = states.pending;
        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        }
    }
}

Now errors thrown during Promise execution like

new Nancy(resolve => { throw new Error(); })

are captured by

reject

. We can also do things like

new Nancy(resolve => resolve(42))

except… it doesn’t do what we want!

this.state

would be changed to

states.resolved

, but we also need

this.value

to be

! Let’s change the

resolve

and

reject

definitions:

const getCallback = state => value => {
    this.value = value;
    this.state = state;
};

const resolve = getCallback(states.resolved);
const reject = getCallback(states.rejected);

We used a Higher-Order-Function,

getCallback

, to avoid repeated code for

resolve

and

reject

. Our

resolve(42)

now works as expected.

Time for the beefier stuff! The infamous “

then

”s. The

then

interface allows a Promise to be chained, which means it should return a Promise. First we create

Nancy.resolve

and

Nancy.reject

syntactic sugars:

class Nancy {
    ...
    static resolve(value) {
        return new Nancy(resolve => resolve(value));
    }

    static reject(value) {
        return new Nancy((_, reject) => reject(value));
    }
}

This allows us to write our

new Nancy(resolve => resolve(42))

Nancy.resolve(42)

. Now let’s see what we expect from then:

// Ignore
let p = Nancy.reject(42)
    .then(() => console.log('why')) // ignored
    .then(() => console.log('you')) // ignored
    .then(() => console.log('ignoring me?!')); // ignored!
// p is a Nancy
// p.state is states.rejected
// p.value is 42

const carry = output => input => {
    console.log(input);
    return output;
};

// Chain
p = Nancy.resolve(0)
    .then(carry(1)) // logs 0
    .then(carry(2)) // logs 1
    .then(carry(3)); // logs 2
// p is a Nancy
// p.state is states.resolved
// p.value is 3

then

has different behaviour in

rejected

and

resolved

states. That means lots of “

if

”s, or… maybe we can do better?

class Nancy {
    constructor(executor) {
        const members = {
            [states.resolved]: {
                state: states.resolved,
                // Chain mechanism
                then: onResolved => Nancy.resolve(onResolved(this.value))
            },
            [states.rejected]: {
                state: states.rejected,
                // Ignore mechanism
                then: _ => this
            },
            [states.pending]: {
                state: states.pending
            }
        };
        const changeState = state => Object.assign(this, members[state]);
        const getCallback = state => value => {
            this.value = value;
            changeState(state);
        };

        const resolve = getCallback(states.resolved);
        const reject = getCallback(states.rejected);
        changeState(states.pending);
        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        }
    }

    static resolve(value) {
        return new Nancy(resolve => resolve(value));
    }

    static reject(value) {
        return new Nancy((_, reject) => reject(value));
    }
}

As you see, no

if

! We’ve implemented a mechanism for “shifting the gear”, our state machine behaves differently on each gear. That

changeState

function in line 18 does what all those condition checks would do for us, voila!

One caveat:

Nancy.resolve(42).then(() => { throw new Error(); })

. This should result in a

rejected

state, but throws the error instead. Not to worry! Our friends at TC39 have a proposal that we are just going to implement. Introducing

Nancy.try

class Nancy {
    constructor(executor) {
        const tryCall = callback => Nancy.try(() => callback(this.value));
        const members = {
            [states.resolved]: {
                ...
                then: trycall
            },
            ...
        };
        ...
    }
    ...

    static try(callback) {
        return new Nancy(resolve => resolve(callback()));
    }
}

You may think implementing

catch

is about as much hassle. Think again! It’s as easy as inverting

then

[states.resolved]: {
    ...
    then: tryCall,
    catch: _ => this
},
[states.rejected]: {
    ...
    then: _ => this,
    catch: tryCall
}

Now this works:

const anything = () => {
    throw new Error('I can be anything because I should never get called!');
};
const throwSomethingWrong = () => {
    console.log('not ignored!');
    throw new Error('Something went wrong...');
};

const p = Nancy.reject(42)
    .catch(value => value) // resolves
    .catch(anything) // ignored
    .catch(anything) // ignored
    .then(value => console.log(value)) // logs 42
    .then(throwSomethingWrong) // logs not ignored!
    .catch(throwSomethingWrong) // logs not ignored!
    .catch(() => 24); // resolves
// p is a Nancy
// p.state is states.resolved
// p.value is 24

Two other things that we should fix:

let p = new Nancy((resolve, reject) => {
    resolve(42);
    reject(24); // ignored
    resolve(); // ignored
});

p
    .then(value => Nancy.reject(value)) // rejects
    .catch(value => console.log(value)); // logs 42

p = Nancy.reject(Nancy.resolve(42));
// p.state is states.rejected
// p.value is a Nancy resolved to 42

Ignoring subsequent calls to

resolve

and

reject

and unpacking a Promise

value

resolve

(and not

reject

). We address both these issues in

getCallback

by moving the previous

value

assignment and

changeState

call to a new function,

apply

const apply = (value, state) => {
    // Ignore subsequent calls to resolve and reject
    if (this.state === states.pending) {
        this.value = value;
        changeState(state);
    }
};

const getCallback = state => value => {
    // Unpack on resolve
    if (value instanceof Nancy && state === states.resolved) {
        value.then(value => apply(value, states.resolved));
        value.catch(value => apply(value, states.rejected));
        // Either 'then' or 'catch' will happen here, not both
        // No need for more ifs!
    } else {
        apply(value, state);
    }
};

Well, no escaping the “

if

”s this time I’m afraid… until the day that

match

comes around!

It’s probably time to acknowledge the elephant in the room. Where’s async in all this? Right, maybe you think it’s going to be a lot of work? Save for a good laugh (spoiler: we are 7 lines away)!

In order to create an async scenario, we first write the

Nancy

version of the popular

delay

function:

const delay = milliseconds => new Nancy(resolve => setTimeout(resolve, milliseconds));
const logThenDelay = milliseconds => total => {
    console.log(`${total / 1000.0} seconds!`);
    return delay(milliseconds)
        .then(() => total + milliseconds);
};

logThenDelay(500)(0) // logs 0 seconds!
    .then(logThenDelay(500)) // after 0.5 seconds, logs 0.5 seconds!
    .then(logThenDelay(500)) // after 1 second, logs 1 seconds!
    .then(logThenDelay(500)); // after 1.5 seconds, logs 1.5 seconds!

We should also accommodate for multiple

then

and

catch

on a single Promise:

let p = delay(500);
p.then(() => console.log('1st then!')); // after 0.5 seconds, logs 1st then!
p.then(() => console.log('2nd then!')); // after 0.5 seconds, logs 2nd then!
p.then(() => console.log('3rd then!')); // after 0.5 seconds, logs 3rd then!

p = p.then(() => Nancy.reject());
p.catch(() => console.log('1st catch!')); // after 0.5 seconds, logs 1st catch!
p.catch(() => console.log('2nd catch!')); // after 0.5 seconds, logs 2nd catch!
p.catch(() => console.log('3rd catch!')); // after 0.5 seconds, logs 3rd catch!

The problem is, our code knows how to handle

then

and

catch

on a

resolved

rejected

state, we just need to hold up until the state arrives there. Our bigger problem is that we need to return a Promise right now! Hmm, well, those are not really problems, they are actually the solution! Let’s do what we just said:

class Nancy {
    constructor(executor) {
        ...
        const laterCalls = [];
        const callLater = getMember => callback => new Nancy(resolve => laterCalls.push(() => resolve(getMember()(callback))));
        const members = {
            ...
            [states.pending]: {
                ...
                then: callLater(() => this.then),
                catch: callLater(() => this.catch)
            }
        };
        ...
        const apply = (value, state) => {
            if (this.state === states.pending) {
            ...
                for (const laterCall of laterCalls) {
                    laterCall();
                }
            }
        };
        ...
    }
    ...
}

We cached both the call to

then

catch

and returned Promise’s

resolve

in a

laterCall

. We call these at the end of

apply

later. Boom!

We may not be particularly proud of the verbose code of our

callLater

definition. Not to worry though, one day we will re-write it with the

pipe

syntax.

If you find

callLater

confusing, you can read more here on how it works with an example.

Here’s our code in its final glory:

const states = {
    pending: 'Pending',
    resolved: 'Resolved',
    rejected: 'Rejected'
};

class Nancy {
    constructor(executor) {
        const tryCall = callback => Nancy.try(() => callback(this.value));
        const laterCalls = [];
        const callLater = getMember => callback => new Nancy(resolve => laterCalls.push(() => resolve(getMember()(callback))));
        const members = {
            [states.resolved]: {
                state: states.resolved,
                then: tryCall,
                catch: _ => this
            },
            [states.rejected]: {
                state: states.rejected,
                then: _ => this,
                catch: tryCall
            },
            [states.pending]: {
                state: states.pending,
                then: callLater(() => this.then),
                catch: callLater(() => this.catch)
            }
        };
        const changeState = state => Object.assign(this, members[state]);
        const apply = (value, state) => {
            if (this.state === states.pending) {
                this.value = value;
                changeState(state);
                for (const laterCall of laterCalls) {
                    laterCall();
                }
            }
        };

        const getCallback = state => value => {
            if (value instanceof Nancy && state === states.resolved) {
                value.then(value => apply(value, states.resolved));
                value.catch(value => apply(value, states.rejected));
            } else {
                apply(value, state);
            }
        };

        const resolve = getCallback(states.resolved);
        const reject = getCallback(states.rejected);
        changeState(states.pending);
        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        }
    }

    static resolve(value) {
        return new Nancy(resolve => resolve(value));
    }

    static reject(value) {
        return new Nancy((_, reject) => reject(value));
    }

    static try(callback) {
        return new Nancy(resolve => resolve(callback()));
    }
}

Hey, we did it! A functional

Promise

named

Nancy

in exactly 70 lines of clean code. Hooray!

Another good exercise is to implement

Nancy.all

and

Nancy.race

, but I leave that to the beloved reader. You can find the code for this article in this repository. Hope it‘s been an interesting read!

Let me know your feedback in the comments section.