Detailed explanation of indexOf method in JavaScript array

I recently encountered a small problem with the code and I will simplify it into a small example to show you.

You will be rewarded if you pay attention in the end. Children's shoes with a solid foundation can jump directly to array type and use.

Speaking of indexOf, everyone is familiar with it. It is especially commonly used when judging whether a string contains a substring (a useful tool for students who are not familiar with regular expressions).

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 appearing in the string respectively Location. 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 worry, let’s give another example

let numStr = '2016';

numStr.indexOf('2');  //0
numStr.indexOf(2);  //0

There is a small point here that indexOf can do simple type conversion. Convert the number into a string '2' and then execute it.

Use of Number type

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

let num = 2016;

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

above, 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 is OK. People who are obsessed with code like the use of the third

Array type

Everyone, cheer up, the big boss is coming.

Everyone is familiar with the array method, but they have overlooked that the array has 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 examples are 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 index is matched and returned.

• arr.indexOf(‘o’) outputs -1 because this method does not perform an indexOf match 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 of the match until the match is reached, rather than returning the index of all matches.

• 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, Strict equals (===) are used here. 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 accumulation of small knowledge points is not a topic for in-depth discussion, so the second parameter of indexOf() is not explained here. I believe everyone knows the role of the second parameter. If you know, you can look at String.prototype.indexOf() here, and then take a look at the second parameter in conjunction with the link to the array above.

The above is the detailed explanation of the indexOf method in JavaScript arrays. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!