indexOf method in JavaScript array

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 the positions where o and n appear in the string respectively. 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! He looked helpless.

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 a simple type conversion, convert the number into the 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 there is no, 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 for the number type? Then convert it to String, follow the above example to 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

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 bit longer than the third way of writing. Haha, in fact, both are fine. People who are obsessed with code prefer the use of the third

Array type

Everyone, cheer up, the big boss is coming.

Everyone is very familiar with array methods, but they ignore that arrays have the indexOf method (my personal feeling).

It’s just talk without practice. What problems did you encounter 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 the indexOf match again on a per-element basis.

arr.indexOf('2016') outputs 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 trap 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 that strict equality (===) is 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 don’t know, you can see here String.prototype .indexOf(), and then take a look at the second parameter combined with the link to the array above.