js判断字符串长度的办法(js包含字符串判断)

includes 函数 语法

<span>str</span><span>.includes</span>(<span>searchString</span><span>[, position]</span>)

includes() 方法用于判断一个字符串是否包含在另一个字符串中，根据情况返回 true 或 false。

includes() 方法是区分大小写的，例如，下面的表达式会返回 false ：

<span>\'Blue Whale\'</span>.includes(<span>\'blue\'</span>); <span>//</span> returns <span>false</span>

兼容补丁(polyfill)

<span>if</span> (!<span>String</span>.prototype.includes) {  <span>String</span>.prototype.includes = <span><span>function</span>(<span>search, start</span>) </span>{    <span>\'use strict\'</span>;    <span>if</span> (<span>typeof</span> start !== <span>\'number\'</span>) {      start = <span>0</span>;    }    <span>if</span> (start + search.length > <span>this</span>.length) {      <span>return</span> <span>false</span>;    } <span>else</span> {      <span>return</span> <span>this</span>.indexOf(search, start) !== <span>-1</span>;    }  };}

示例

<span>var</span> str = <span>\'To be, or not to be, that is the question.\'</span>;<span>console</span>.log(str.includes(<span>\'To be\'</span>));       <span>// true</span><span>console</span>.log(str.includes(<span>\'question\'</span>));    <span>// true</span><span>console</span>.log(str.includes(<span>\'nonexistent\'</span>)); <span>// false</span><span>console</span>.log(str.includes(<span>\'To be\'</span>, <span>1</span>));    <span>// false</span><span>console</span>.log(str.includes(<span>\'TO BE\'</span>));       <span>// false</span>

indexOf 函数语法

<span>str</span><span>.indexOf</span>(<span>searchValue</span> <span>[, fromIndex]</span>)

参数

searchValue

要被查找的字符串值。

如果没有提供确切地提供字符串，searchValue 会被强制设置为 “undefined”， 然后在当前字符串中查找这个值。

举个例子：’undefined’.indexOf() 将会返回0，因为 undefined 在位置0处被找到，但是 ‘undefine’.indexOf() 将会返回 -1 ，因为字符串 ‘undefined’ 未被找到。

fromIndex 可选

数字表示开始查找的位置。可以是任意整数，默认值为 0。

如果 fromIndex 的值小于 0，或者大于 str.length ，那么查找分别从 0 和str.length 开始。

举个例子，’hello world’.indexOf(‘o’, -5) 返回 4 ，因为它是从位置0处开始查找，然后 o 在位置4处被找到。另一方面，’hello world’.indexOf(‘o’, 11) （或 fromIndex 填入任何大于11的值）将会返回 -1 ，因为开始查找的位置11处，已经是这个字符串的结尾了。

示例

var anyString = \"Brave new world\";console.log(\"The index of the first w from the beginning is \" + anyString.indexOf(\"w\"));// logs 8console.log(\"The index of the first w from the <span>end</span> <span>is</span> <span>\" + anyString.lastIndexOf(\"</span>w<span>\"));// logs 10console.log(\"</span>The <span>index</span> <span>of</span> <span>\'new\'</span> <span>from</span> the <span>beginning</span> <span>is</span> <span>\" + anyString.indexOf(\"</span><span>new</span><span>\"));// logs 6console.log(\"</span>The <span>index</span> <span>of</span> <span>\'new\'</span> <span>from</span> the <span>end</span> <span>is</span> <span>\" + anyString.lastIndexOf(\"</span><span>new</span><span>\"));// logs 6</span>