Analysis of indexOf method in JS array (with examples)

The content of this article is about the analysis of the indexOf method in JS arrays (with examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

In the past two days, I have been helping a friend work on a project at home. The indexOf method of the array was used in the project, and I found an article that felt very good. I organized it by the way to prevent the link from being lost.

I believe everyone is familiar with indexOf. It is particularly commonly used when judging whether a string contains substrings. It is a useful tool for students who are not familiar with regular expressions. This article uses an example to talk about the indexOf method based on a problem I encountered recently. This article is an accumulation of small knowledge points and is not a topic for in-depth discussion. Therefore, the second parameter of indexOf() is not explained here. I believe everyone knows the role of the second parameter.

Usage of String type

Review the well-known string usage, for example

let str = 'orange';
 
str.indexOf('o'); //0
str.indexOf('n'); //3
str.indexOf('c'); //-1

Here 0 and 3 are o and n respectively The position at which it occurs in the string. The starting index is 0. And -1 means no match.
Someone once asked me why -1 is not null or undefined. Ask the person who made the rules! A look of helplessness.
Everyone feels that there is no highlight here, don’t rush and let’s give another example

let numStr = '2016';
 
numStr.indexOf('2'); //0
numStr.indexOf(2); //0

There is a small point here that indexOf will do simple type conversion, convert numbers into strings '2' and then execute .

Use of Number type

You may wonder whether the number type has an indexOf method because it will perform implicit conversion! Let me tell you clearly that no, the above example

let num = 2016;
 
num.indexOf(2); //Uncaught TypeError: num.indexOf is not a function

must use the indexOf method on the number type? Then convert it into a string, and then write

//二逼青年的写法
num = '2016';
num.indexOf(2); //0
 
//普通青年的写法
num.toString().indexOf(2); //0
 
//文艺青年的写法
('' + num).indexOf(2); //0

in the above example. The first way of writing is simple and direct, and it is not impossible for known shorter numbers. But what should we do when the num variable changes for different data?

The second way of writing is the most commonly used, but it is a little longer than the third way of writing. Haha, actually both are fine. People who are obsessed with code prefer the third one √ .

Usage of array type

Everyone cheer up, the big boss is coming.
Everyone is familiar with array methods, but they ignore that arrays have the indexOf method (my personal feeling).
Talking nonsense without practicing, what problems did you encounter, and what are the key points to pay attention to?

let arr = ['orange', '2016', '2016'];
 
arr.indexOf('orange'); //0
arr.indexOf('o'); //-1
 
arr.indexOf('2016'); //1
arr.indexOf(2016); //-1

The examples are not broken down in detail here. Four use cases are enough to illustrate the problem.

arr.indexOf('orange') Output 0 because 'orange' is the 0th element of the array, the index is matched and returned.
arr.indexOf('o') Output -1 because this method does not perform indexOf matching again on a per-element basis.
arr.indexOf('2016') Output 1 Because this method returns the following list of the first array element from the beginning until the match is reached, rather than returning all matching indexes.
arr.indexOf(2016) Output -1 Note: No implicit type conversion will be done here.

Now that the pit has been discovered, we might as well get to the bottom of it. Go to the MDN official website to find out. Friends who are interested in this topic can jump directly to Array.prototype.indexOf().

For those who just want to know more, here is the official Description.

indexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator).

Clear at a glance,used here Is strictly equal to (===) . Please pay more attention when making similar judgments. Don't make the mistake of thinking that numbers will be converted to strings, and strings will not be converted to numbers.

Summary

The above is the entire content of this article. I hope the content of this article can bring some help to everyone's study or work. If you have any questions, You can leave messages to communicate.

• String: indexOf will perform type conversion on the number 0 and convert it to the string '0'.

• Number: There is no indexOf method. Need to convert numeric type to string. There are three methods. Direct string, string toString, string concatenation.

• Array: Arrays also have the indexOf method, but when doing type judgment, strict equality is used, that is, ===.

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