JS understanding the weird parts 4-51 ~ 4-52 Functional Programming

4-51 Functional Programming

• 利用first class function 特性，將function當成參數傳入另一個function 或回傳一個function
• 盡量減少funciton side-effect，不更改原變數，不依賴全域變數 -> 每次function執行，結果都要相同

selfdefined map function

function mapForEach(arr, fn) {

    var newArr = [];
    for (var i=0; i < arr.length; i++) {
        newArr.push(
            fn(arr[i])
        )
    };

    return newArr;
}

var arr1 = [1,2,3];
console.log(arr1);


var arr2 = mapForEach(arr1, function(item) {
   return item * 2;
});
console.log(arr2);


var arr3 = mapForEach(arr1, function(item) {
   return item > 2;
});
console.log(arr3);


var checkPastLimit = function(limiter, item) {
    return item > limiter;
}
var arr4 = mapForEach(arr1, checkPastLimit.bind(this, 1));
console.log(arr4);


var checkPastLimitSimplified = function(limiter) {
    return function(limiter, item) {
        return item > limiter;
    }.bind(this, limiter);
};

var arr5 = mapForEach(arr1, checkPastLimitSimplified(1));
console.log(arr5);

4-51 Functional Programming

• Read code of open source library
• underscore.js
• lodash.js