Briefly talk about the indexOf method in JS array

Preface

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

Let’s 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 respectively and the position where n 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 sees nothing interesting here, don’t worry, here is 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 the number to the string '2' and then execute.

Use of Number type

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

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

, do you have to 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, in fact, all are possible. People who are obsessed with code prefer the third type ✅

Array type use

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 have you encountered, and what should you 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 example is not broken down in detail here. Four use cases are enough to illustrate the problem.

arr.indexOf(‘orange’) outputs 0 because ‘orange’ is the 0th element of the array, and the subscript is matched and returned.

arr.indexOf(‘o’) outputs -1 because this method does not perform indexOf matching again on a per-element basis.

arr.indexOf(‘2016’) outputs 1 because this method returns the table below of the first array element from the beginning until the match is reached, rather than returning all matching subscripts.

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).

It is clear at a glance, what is 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 a message to communicate.

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