JavaScript parseInt() and Number() difference case study

Learning objectives:

The two functions parseInt() and Number() are most often used to convert a string into a data type, so what are the differences between them?

You will learn:

The parseInt() function parses the given string into an integer in the specified radix.
parseInt(string, radix)
The second parameter indicates the base used. We usually use decimal, but there may also be octal or hexadecimal. In order to avoid parsing errors for strings starting with "0" and "0x", various JavaScript programming specifications require that the value of the second parameter must be clearly given, such as parseInt("123",10).

parseInt('16', 8) = 14
parseInt('10', 8) = 8

parseInt('16', 10) = 16
parseInt('10', 10) = 10

parseInt('16', 16) = 22
parseInt('10', 16) = 16

parseInt parses the string into an integer from the beginning. When it encounters a character that cannot be parsed, it returns the parsed integer part. If the first character cannot be parsed, it returns NaN directly.

Number() can be used to perform type conversions when the new operator is not used. If it cannot be converted to a number, NaN is returned. For example, for "123a", parseInt() returns 123, while Number() returns NaN. Different types of strings are converted differently using these two functions:

// When the string is composed of numbers, they convert to the same number without any difference let numStr = '123'
console.log(parseInt(numStr)) //123
console.log(Number(numStr)) //123

// When the string is composed of letters let numStr = 'abc'
console.log(parseInt(numStr)) //NaN
console.log(Number(numStr)) //NaN

// When the string is composed of numbers and letters let numStr = '123a'
console.log(parseInt(numStr)) //123
console.log(Number(numStr)) //NaN

// When the string is composed of 0 and digits let numStr = '0123'
console.log(parseInt(numStr)) //123
console.log(Number(numStr)) //123

// **When the string contains a decimal point**
let numStr = '123.456'
console.log(parseInt(numStr)) //123
console.log(Number(numStr)) //123.456

// **When the string is null**
let numStr = null
console.log(parseInt(numStr)) //NaN
console.log(Number(numStr)) //0

// **When the string is '' (empty)**
let numStr = ''
console.log(parseInt(numStr)) //NaN
console.log(Number(numStr)) //0

Learning summary:

1. When the string is composed of numbers, the numbers they convert are the same without any difference; if the string does not contain numbers but only letters, both methods will just return NaN results; when the string is composed of 0 and numbers, all numbers except 0 will be parsed;

2 When the string is composed of numbers and letters ① The letter is at the beginning, and both methods return NaN results ② The letter is not at the beginning. The Number method returns NaN, and the pareseInt method returns the data before the letter

3 parseInt converts non-String values ​​into String type before operation 4 For the rest of the details, refer to the above case

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