JavaScript

JSON

JSON.parse

将字符串转化为JSON

const json = '{"result":true, "count":42}';
const obj = JSON.parse(json);

console.log(obj.count);
// expected output: 42

console.log(obj.result);
// expected output: true

JSON.stringify

将JSON转化为字符串

console.log(JSON.stringify({ x: 5, y: 6 }));
// expected output: "{"x":5,"y":6}"

console.log(JSON.stringify([new Number(3), new String('false'), new Boolean(false)]));
// expected output: "[3,"false",false]"

console.log(JSON.stringify({ x: [10, undefined, function(){}, Symbol('')] }));
// expected output: "{"x":[10,null,null,null]}"

console.log(JSON.stringify(new Date(2006, 0, 2, 15, 4, 5)));
// expected output: ""2006-01-02T15:04:05.000Z""

Date

处理时间的函数，平时就用个时间戳new Date().getTime()剩下得几乎用不到，文档链接

Math

数学用函数，只整理常用的，其余看文档，文档链接

Math.abs

取绝对值

function difference(a, b) {
  return Math.abs(a - b);
}

console.log(difference(3, 5));
// expected output: 2

console.log(difference(5, 3));
// expected output: 2

console.log(difference(1.23456, 7.89012));
// expected output: 6.6555599999999995

Math.ceil

返回大于等于给定量的最小整数

console.log(Math.ceil(.95));
// expected output: 1

console.log(Math.ceil(4));
// expected output: 4

console.log(Math.ceil(7.004));
// expected output: 8

console.log(Math.ceil(-7.004));
// expected output: -7

Math.floor

返回小于给定量的最大整数

console.log(Math.floor(5.95));
// expected output: 5

console.log(Math.floor(5.05));
// expected output: 5

console.log(Math.floor(5));
// expected output: 5

console.log(Math.floor(-5.05));
// expected output: -6

Math.max

返回给定值最大的一个

console.log(Math.max(1, 3, 2));
// expected output: 3

console.log(Math.max(-1, -3, -2));
// expected output: -1

const array1 = [1, 3, 2];

console.log(Math.max(...array1));
// expected output: 3

Math.min

返回给定值最小的那个

console.log(Math.min(2, 3, 1));
// expected output: 1

console.log(Math.min(-2, -3, -1));
// expected output: -3

const array1 = [2, 3, 1];

console.log(Math.min(...array1));
// expected output: 1

Math.pow

返回给定的量指定次数的幂

console.log(Math.pow(7, 3));
// expected output: 343

console.log(Math.pow(4, 0.5));
// expected output: 2

console.log(Math.pow(7, -2));
// expected output: 0.02040816326530612
//                  (1/49)

console.log(Math.pow(-7, 0.5));
// expected output: NaN

Math.random

返回一个[0,1)之间随机的数

function getRandomInt(max) {
  return Math.floor(Math.random() * Math.floor(max));
}

console.log(getRandomInt(3));
// expected output: 0, 1 or 2

console.log(getRandomInt(1));
// expected output: 0

console.log(Math.random());
// expected output: a number between 0 and 1

Math.round

返回给定数的舍入后的整数

console.log(Math.round(0.9));
// expected output: 1

console.log(Math.round(5.95), Math.round(5.5), Math.round(5.05));
// expected output: 6 6 5

console.log(Math.round(-5.05), Math.round(-5.5), Math.round(-5.95));
// expected output: -5 -5 -6

Math.sign

返回给定数的符号，正1负-1零0

console.log(Math.sign(3));
// expected output: 1

console.log(Math.sign(-3));
// expected output: -1

console.log(Math.sign(0));
// expected output: 0

console.log(Math.sign('-3'));
// expected output: -1

Math.sqrt

返回给定数的平方根

function calcHypotenuse(a, b) {
  return (Math.sqrt((a * a) + (b * b)));
}

console.log(calcHypotenuse(3, 4));
// expected output: 5

console.log(calcHypotenuse(5, 12));
// expected output: 13

console.log(calcHypotenuse(0, 0));
// expected output: 0

Number

数字类型，文档链接

Number.isInteger

判定给定值是否是整数

function fits(x, y) {
  if (Number.isInteger(y / x)) {
    return 'Fits!';
  }
  return 'Does NOT fit!';
}

console.log(fits(5, 10));
// expected output: "Fits!"

console.log(fits(5, 11));
// expected output: "Does NOT fit!"

Number.isFinite

判定给定值是否是有限数

console.log(Number.isFinite(1 / 0));
// expected output: false

console.log(Number.isFinite(10 / 5));
// expected output: true

console.log(Number.isFinite(0 / 0));
// expected output: false

number.toPrecision

方法返回表示Number对象到指定的精度的字符串

function precise(x) {
  return Number.parseFloat(x).toPrecision(4);
}

console.log(precise(123.456));
// expected output: "123.5"

console.log(precise(0.004));
// expected output: "0.004000"

console.log(precise('1.23e+5'));
// expected output: "1.230e+5"

Number.MAX_VALUE

js中最大正整数

Number.MAX_SAFE_INTEGER

js最大安全整数

Number.MIN_SAFE_INTEGER

js最小安全整数

Number.MIN_VALUE

js最小正整数值

Map

是ES6新增的，相较于对象就是属性名可以为任意值

map.@@iterator

很明显，map是可以迭代的

const map1 = new Map();

map1.set('0', 'foo');
map1.set(1, 'bar');

const iterator1 = map1[Symbol.iterator]();

for (let item of iterator1) {
  console.log(item);
}
// expected output: Array ["0", "foo"]
// expected output: Array [1, "bar"]

map.clear

清空map

const map1 = new Map();

map1.set('bar', 'baz');
map1.set(1, 'foo');

console.log(map1.size);
// expected output: 2

map1.clear();

console.log(map1.size);
// expected output: 0

map.delete

移出map中的某个元素

const map1 = new Map();
map1.set('bar', 'foo');

console.log(map1.delete('bar'));
// expected result: true
// (true indicates successful removal)

console.log(map1.has('bar'));
// expected result: false

map.entries

返回一个可迭代的对象，每次返回键值对的数组

const map1 = new Map();

map1.set('0', 'foo');
map1.set(1, 'bar');

const iterator1 = map1.entries();

console.log(iterator1.next().value);
// expected output: ["0", "foo"]

console.log(iterator1.next().value);
// expected output: [1, "bar"]

map.forEach

对每个键值对都执行一次某个操作

function logMapElements(value, key, map) {
  console.log(`m[${key}] = ${value}`);
}

new Map([['foo', 3], ['bar', {}], ['baz', undefined]])
  .forEach(logMapElements);

// expected output: "m[foo] = 3"
// expected output: "m[bar] = [object Object]"
// expected output: "m[baz] = undefined"

map.get

获取map的某个键对应的值，没有返回undefined

const map1 = new Map();
map1.set('bar', 'foo');

console.log(map1.get('bar'));
// expected output: "foo"

console.log(map1.get('baz'));
// expected output: undefined

map.has

查询某个键是否存在

const map1 = new Map();
map1.set('bar', 'foo');

console.log(map1.has('bar'));
// expected output: true

console.log(map1.has('baz'));
// expected output: false

map.keys

返回一个可迭代的对象，值是map的键

const map1 = new Map();

map1.set('0', 'foo');
map1.set(1, 'bar');

const iterator1 = map1.keys();

console.log(iterator1.next().value);
// expected output: "0"

console.log(iterator1.next().value);
// expected output: 1

map.set

设置一个键值对到map里边

const map1 = new Map();
map1.set('bar', 'foo');

console.log(map1.get('bar'));
// expected output: "foo"

console.log(map1.get('baz'));
// expected output: undefined

map.size

返回map的大小，也就是多少个元素

const map1 = new Map();

map1.set('a', 'alpha');
map1.set('b', 'beta');
map1.set('g', 'gamma');

console.log(map1.size);
// expected output: 3

map.values

返回一个可迭代对象，值是map的值

const map1 = new Map();

map1.set('0', 'foo');
map1.set(1, 'bar');

const iterator1 = map1.values();

console.log(iterator1.next().value);
// expected output: "foo"

console.log(iterator1.next().value);
// expected output: "bar"

Set

和Map的api是一样的，不过不允许存在重复的值，所以可以用来进行数组去重，文档链接

语言 js js 基础

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