forEachAsync

forEach.js

// https://2ality.com/2016/10/asynchronous-iteration.html

async function forEach(object, callback) {
    if (Array.isArray(object)) {
        for (let prop = 0; prop < object.length; ++prop) {
            await callback(object[prop], prop)
        }
    } else {
        for (const prop in object) {
            await callback(object[prop], prop)
        }
    }
}

// Usage:
await forEach(['A','B','C','D'], async (item, index) => {
    await fetch('/whatever')
})
await forEach({A:1, B:2}, async (value, prop) => {
    await fetch('/whatever')
})

forEachAsync.js

function forEachAsync( arr, b, c ) {

    var onNext = b;
    var onFinish = c;
    
    if ( typeof arr != 'object' ) {
        onFinish = b;
        onNext = arr;
        arr = this;
    }

    var length = arr.length;
    (function loop(index) {

        if ( index < length )
            onNext( loop.bind(this, index+1), arr[index], index );
        else if ( onFinish )
            onFinish();

    })(0);

}

Array.prototype.forEachAsync = forEachAsync;

// USING
forEachAsync(['A','B','C','D'], (next, item, index) => {
    console.log(item, index);
    next();
}, () => {
    console.log('END');
});

// USING AS PROTOTYPE
['E','F','G','H'].forEachAsync((next, item, index) => {
    console.log(item, index);
    next();
}, () => {
    console.log('END');
});