Summary of commonly used string usage methods in Javascript

This article mainly introduces the common usage methods related to string in Javascript. Has very good reference value. Let’s take a look at it with the editor

I was just reading the Javascript Rhino book and saw the section on strings. I don’t have much exposure to this aspect in my daily work, so I thought I would sort it out in case I need it. .

One of the built-in functions of Javascript is string concatenation. If you use '+' to connect two numbers, it means adding the two numbers. But if used in a string, it means that the second character is added after the first character.

var num=1+2;
console.log(num);
var msg='hello'+'world';
console.log(msg);

For strings, in addition to the length attribute, there are many other useful attributes, such as:

var str='hello,world';
console.log(str.length);
console.log(str.charAt(0)); //第一个字符 
console.log(str.charAt(str.length-1)); // 最后一个字符 
//substring(starti,endi), 截取 字符串 起始startistartii， 到endi结束，且含头不含尾，没有
// 如果第二个参数没有， 就默认截取到最后一个。  
console.log(str.substring(1,4));
console.log(str.substring(1));
//用法同上，为负数时，就是倒数开始算，一个参数含义是倒数几个。
console.log(str.slice(1,4));
console.log(str.slice(-3));
//字符‘l'首次出现的位置 
console.log(str.indexOf('l'));
//字符‘l'最后一次出现的位置 
console.log(str.lastIndexOf('l'));
//在位置下标3之后，首次出现的位置 
console.log(str.indexOf('l',3));
//用‘，'分割为字符串
console.log(str.split(','));
// 把str中的小写h换成大写H
console.log(str.replace('h','H'));
//讲字符串转化为大写
console.log(str.toUpperCase());

Tips: The string itself is fixed in JavaScript. The above methods will return a new string value and will not affect the value of str itself.

It is worth noting that in ES6 , adds many new methods to strings, such as:

var s = 'Hello world!';
// 返回布尔值，表示参数字符串是否在源字符串中的头部
console.log(s.startsWith('Hello')) // true
//endsWith()：返回布尔值，表示参数字符串是否在源字符串的尾部
console.log(s.endsWith('!')) // true
// includes()返回布尔值，表示是否找到了参数字符串
console.log(s.includes('o')) // true

The above three methods all support the second parameter, which indicates the position to start search

var s = 'Hello world!';
console.log(s.startsWith('world', 6)) // true
console.log(s.endsWith('Hello', 5))// true
console.log(s.includes('Hello', 6)) // false

Tip: endsWith behaves differently than the other two methods. It targets the first n characters, while the other two methods target from the nth position until the end of the string.
The repeat method returns a new string, which means repeating the original string n times.

console.log('x'.repeat(3)) // "xxx"
console.log('hello'.repeat(2)) // "hellohello"
console.log('na'.repeat(0)) // ""

If the parameter is a decimal, it will be rounded.

console.log('na'.repeat(2.9)) // "nana"
//如果repeat的参数是负数或者Infinity，会报错。
console.log('na'.repeat(Infinity))
// RangeError
console.log('na'.repeat(-1))
// RangeError

However, if the parameter is a decimal between 0 and -1, it is equivalent to 0, because the rounding operation will be performed first. A decimal between 0 and -1 is equal to -0 after rounding, and repeat is regarded as 0.

console.log('na'.repeat(-0.9)) // ""
//参数NaN等同于0
console.log('na'.repeat(NaN)) // "
//如果repeat的参数是字符串，则会先转换成数字
console.log('na'.repeat('na')) // ""
console.log('na'.repeat('3')) // "nanana"

ES2017 introduces the function of string completion length. If a string is not long enough, it will be completed at the head or tail. padStart() is used for head completion, padEnd() is used for tail completion

//padStart和padEnd一共接受两个参数，第一个参数用来指定字符串的最小长度，第二个参数是用来补全的字符串。
console.log('x'.padStart(5, 'ab')) // 'ababx'
console.log('x'.padStart(4, 'ab')) // 'abax'
console.log('x'.padEnd(5, 'ab')) // 'xabab'
console.log('x'.padEnd(4, 'ab')) // 'xaba'
//如果原字符串的长度，等于或大于指定的最小长度，则返回原字符串。
console.log('xxx'.padStart(2, 'ab')) // 'xxx'
console.log('xxx'.padEnd(2, 'ab')) // 'xxx'
//如果用来补全的字符串与原字符串，两者的长度之和超过了指定的最小长度，则会截去超出位数的补全字符串。
consoe.log('abc'.padStart(10, '0123456789'))
// '0123456abc'
//如果省略第二个参数，默认使用空格补全长度。
console.log('x'.padStart(4)) // ' x'
console.log('x'.padEnd(4)) // 'x '
//padStart的常见用途是为数值补全指定位数。下面代码生成10位的数值字符串。
console.log('1'.padStart(10, '0') )// "0000000001"
console.log('12'.padStart(10, '0')) // "0000000012"
console.log('123456'.padStart(10, '0')) // "0000123456"
//另一个用途是提示字符串格式。
console.log('12'.padStart(10, 'YYYY-MM-DD')) // "YYYY-MM-12"
console.log('09-12'.padStart(10, 'YYYY-MM-DD'))// "YYYY-09-12"

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