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最新推荐文章于 2022-09-03 14:48:33 发布

转载最新推荐文章于 2022-09-03 14:48:33 发布 · 255 阅读

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本文介绍了JavaScript中的变量声明方式，包括let、var和const等关键字的使用。同时深入探讨了环境（Environment）、函数(Function)以及控制流(Control flow)的概念，并且讲解了如何使用箭头函数和可选参数等功能来提高代码效率。此外，还涉及到了数据结构如对象和数组的操作方法。

BINDINGS or variables

keyword: let, var, const:

BINDING NAMES:

A binding name may include dollar signs ($) or underscores (_) but no other

punctuation or special characters.

ENVIRONMENT:

        The collection of bindings and their values that exist at a given time is called
    the environment. When a program starts up, this environment is not empty. It
    always contains bindings that are part of the language standard, and most of the
    time, it also has bindings that provide ways to interact with the surrounding
    system. For example, in a browser, there are functions to interact with the
    currently loaded website and to read mouse and keyboard input.

FUNCTIONS:

A function is a piece of program wrapped in a value .

Control flow :

C类似： if whiel, do-while, for-loops, break, continue, switch-case

Functions as values

    let launchMissiles = function() {
        missileSystem.launch("now");
    }; 

    Declaration notation (没有顺序)

    function square(x) {
        return x * x;
    }

Arrow functions

const square1 = (x) => { return x * x; };

Optional Arguments

    function minus(a, b) {
    if (b === undefined) return -a;
        else return a - b;
    }

Rest parameters

 function max(...numbers) {

        let result = -Infinity;
        for (let number of numbers) {
        if (number > result) result = number;
        }
            return result;
        }
    console.log(max(4, 1, 9, -2));
    // → 9

function power(base, exponent = 2) {

    let result = 1;
    for (let count = 0; count < exponent; count++) {
        result *= base;
    }
        return result;
    }

Data Structures: Objects and Arrays

let anObject = {left: 1, right: 2};
console.log(anObject.left);
// → 1
delete anObject.left;
console.log(anObject.left);
// → undefined
console.log("left" in anObject);
// → false
console.log("right" in anObject);
// → true 

console.log(Object.keys({x: 0, y: 0, z: 2}));
// → ["x", "y", "z"] 

let objectA = {a: 1, b: 2};
Object.assign(objectA, {b: 3, c: 4});
console.log(objectA);
// → {a: 1, b: 3, c: 4}

mutability:

对象的比较是对象id的比较，类似python对象 is 比较

Number , string, boolen, 内置数据类型不可变，

let kim = "Kim";
kim.age = 88;
console.log(kim.age);
// → undefined

DESTRUCTURE

function phi(table) {
return (table[3] * table[0] - table[2] * table[1]) /
Math.sqrt((table[2] + table[3]) *
(table[0] + table[1]) *
(table[1] + table[3]) *
(table[0] + table[2]));
} 

function phi([n00, n01, n10, n11]) {
return (n11 * n00 - n10 * n01) /
Math.sqrt((n10 + n11) * (n00 + n01) *
(n01 + n11) * (n00 + n10));
}

JSON

JSON.stringify(object) ， JSON.parse(string)