Detailed explanation of padStart() and padEnd() methods in JavaScript

ES2017 introduces the function of string completion length. If a string is not long enough, it will be completed at the head or tail. padStart() is used for head completion , padEnd() is used for tail completion.

const string = 'hi';

string.padStart(3, 'c'); // "chi"

string.padEnd(4, 'l'); // "hill"

Syntax

string.padStart(<maxLength>, <padString>)

string.padEnd(<maxLength>, <padString>)

Understanding the parameters

padEnd and padStart accept the same parameters.

1. maxLength

The length of the final string.

const result = string.padStart(5);

result.length; // 5

When I saw this, it also took me a while to learn. I always thought maxLength was the number of times the string parameter was repeatedly filled. So I just want to emphasize that this parameter is the target length that the current string needs to be filled to, not the number of times the filling string is repeated . If this value is less than the length of the current string, the current string itself is returned.

Of course, I believe readers are much smarter than me, so I’m sure you don’t have this confusion??

2. padString

padString means padding string. If the string is too long and the padded string length exceeds the target length, only the leftmost part is retained and the other parts are truncated. The default value of this parameter is a space " "(U 0020.

'hi'.padStart(5);

// 等价于
'hi'.padStart(5, ' ');

If you pass in an empty string, nothing will be filled in.

const result = 'hi'.padStart(5, '');

result; // "hi"
result.length; // 2

How to handle other data types

For the second parameter padString, it accepts a string. If we try to pass in other data types to it. It will Call the toString method to force conversion to a string. Let's see what happens when using toString on different value types.

// Number
(100).toString(); // '100'

// Boolean
true.toString();   // 'true'
false.toString();  // 'false'

// Array
['A'].toString(); // 'A'
[''].toString();  // ''

// Object
({}).toString();         // '[object Object]'
({hi: 'hi'}).toString(); // '[object Object]'

With this knowledge, Let's see if we can pass these other value types to padStart (padEnd has the same behavior).

'SAM'.padStart(8, 100);    // '10010SAM'

'SAM'.padStart(8, true);   // 'truetSAM'
'SAM'.padStart(8, false);  // 'falseSAM'

'SAM'.padStart(5, ['']);   // 'SAM'
'SAM'.padStart(5, ['hi']); // 'hiSAM'

'SAM'.padStart(18, {});         // '[object Object]SAM'
'SAM'.padStart(18, {hi: 'hi'}); // '[object Object]SAM'

Handlingundefined

Here is an interesting example. If you force undefined to be converted into a string, you will get a TypeError:

undefined.toString(); // TypeError: Cannot read property 'toString' of undefined

But when we pass undefined as the second parameter to padStart, we will get this:

'SAM'.padStart(10, undefined);
// '       SAM'

So the ## mentioned above #padString The parameters will be forced to be converted into strings using toString. It feels wrong again here??. Let’s take a look at the specification first:

ECMAScript specification: If the padded string is undefined, the padded string will be regulated as a space (0x0020).

undefined , otherwise all other data types passed will be coerced to strings using toString.

maxLength If the value is less than or equal to the length of the current string, then the current string itself is returned.

'hi'.padEnd(2, 'SAM');
// 'hi'

maxLength is less than the length of padString, then padString will be truncated.

'hi'.padEnd(7, 'SAMANTHA');
// 'hiSAMAN'

trim Alias ​​the method has.

• trimLeft is an alias for trimStart
• trimRight is an alias for trimStart

padLeft and padRight, they don't exist. This is also the reason why it is recommended that you do not use trimaliases, so as to ensure consistency in the code base??

console.log('JavaScript'.padStart(15));
console.log('HTML'.padStart(15));
console.log('CSS'.padStart(15));
console.log('Vue'.padStart(15));

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     JavaScript
           HTML
            CSS
            Vue

const bankNumber = '2222 2222 2222 2222';
const last4Digits = bankNumber.slice(-4);

last4Digits.padStart(bankNumber.length, '*');
// ***************2222

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Browser support

padStart and padEnd are Introduced at the same time, so they share similar browser support, except IE, they can be used??

Original address: https://dmitripavlutin.com/replace-all-string-occurrences -javascript/

Author: Dmitri Pavlutin

Translation address: https://segmentfault.com/a/1190000023721944

Introduction to Programming! !

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