How does js know whether a given substring exists?-JS Tutorial-php.cn

In the previous article "How to calculate the position of an element using js", we introduced the method of obtaining the absolute position relative to the browser window and the offset position relative to the parent node or body element. method. This time we continue to learn javascript and introduce the method of determining whether a specified substring exists in a string.

There are many methods in JavaScript to determine whether a specified substring exists, for example, by obtaining the first or last occurrence position of the substring in the string, and then determining whether the occurrence position really exists, that is, whether Has a positive integer value; if so, the specified substring exists.

How to get the first or last occurrence position of a substring in a string? Let’s look at the following example:

var str = "How do you do!"; //获取子串“do”在str字符串中第一次出现的位置 var index=str.indexOf("do"); console.log("'do'在str字符串中第一次出现的位置是："+index);

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Output result:

How does js know whether a given substring exists?

Because the string position starts at 0, not 1; so we The first occurrence position obtained by usingstr.indexOf("do")is 4.

Look at the following example:

var str = "How do you do!"; //获取子串“do”在str字符串中最后一次出现的位置 var lastindex=str.lastIndexOf("do"); console.log("'do'在str字符串中最后一次出现的位置是："+lastindex);

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Output result:

How does js know whether a given substring exists?

##It can be seen that using

str.lastIndexOf( "do")The last occurrence position obtained is 11.

Because the substring "do" exists (and there are multiple ones), both the first occurrence position and the last occurrence position have positive values.

Let’s look at the non-existent situation:

var str = "How do you do!"; //获取子串“do”在str字符串中最后一次出现的位置 var index=str.indexOf("de"); var lastindex=str.lastIndexOf("de"); console.log("'de'在str字符串中第一次出现的位置是："+index); console.log("'de'在str字符串中最后一次出现的位置是："+lastindex);

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Output result:

How does js know whether a given substring exists?

It can be seen that if the substring does not exist, then Whether it is the first occurrence position or the last occurrence position,

-1is returned.

Based on this, let’s implement a function that determines whether the substring exists given a substring:

function a(str1,str2){ var index=str1.indexOf(str2); var lastindex=str1.lastIndexOf(str2); if(index!=-1){ console.log("第一次出现的位置是："+index+" ,因此 子串 “"+str2+"” 是存在的"); }else{ console.log("第一次出现的位置是："+index+" ,因此 子串 “"+str2+"” 是不存在的"); } if(lastindex!=-1){ console.log("最后一次出现的位置是："+lastindex+" ,因此 子串 “"+str2+"” 是存在的"); }else{ console.log("最后一次出现的位置是："+lastindex+" ,因此 子串 “"+str2+"” 是不存在的"); } }

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Set the string and substring to be retrieved and call a(str1,str2 ) function to judge:

var str = "How do you do!"; a(str,"do"); a(str,"de");

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Output result:

How does js know whether a given substring exists?

Okay, that’s all, you can read it if you need it ：

javascript advanced tutorial

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