数组扁平化

function flatten(arr) {
    var result = [];
    for (var i = 0, len = arr.length; i < len; i++) {
        if (Array.isArray(arr[i])) {
            result = result.concat(flatten(arr[i]))
        } else {
            result.push(arr[i])
        }
    }
    return result;
}

Promise.all

Promise.all = function(promiseArr) {
    let index = 0, result = []
    return new Promise((resolve, reject) => {
        promiseArr.forEach((p, i) => {
            Promise.resolve(p).then(val => {
                index++
                result[i] = val
                if (index === promiseArr.length) {
                    resolve(result)
                }
            }, err => {
                reject(err)
            })
        })
    })
}
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Promise.race

Promise.race = function(promiseArr) {
    return new Promise((resolve, reject) => {
        promiseArr.forEach(p => {
            Promise.resolve(p).then(val => {
                resolve(val)
            }, err => {
                rejecte(err)
            })
        })
    })
}
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Promise.allSettled

Promise.allSettled = function(promiseArr) {
    let result = []
        
    return new Promise((resolve, reject) => {
        promiseArr.forEach((p, i) => {
            Promise.resolve(p).then(val => {
                result.push({
                    status: 'fulfilled',
                    value: val
                })
                if (result.length === promiseArr.length) {
                    resolve(result) 
                }
            }, err => {
                result.push({
                    status: 'rejected',
                    reason: err
                })
                if (result.length === promiseArr.length) {
                    resolve(result) 
                }
            })
        })  
    })   
}
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