Introduction to value type conversion in JavaScript_javascript tips

When performing operations such as -, *, /, ==, != in JavaScript, if the value types on the left and right sides of the operator are inconsistent with the expected type, JavaScript will convert the values ​​on both sides of the operator into the expected type. Do the operation again. When the expected value type is string, JavaScript will convert the value to string; when the expected value type is number, JavaScript will convert the value to number (if it cannot be converted to a numerical value, NaN will be returned). For example:

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console.log(10 "cats");//10 cats
console.log(10 * " cats");//NaN, " cats" will be converted to NaN
console.log(10 "2");//102
console.log(10 - "2");//8
console.log(10 / "2");//5
console.log(10 * "2");//20
console.log(10 * " 2");//20
console.log("10" * "2");//20

Value type conversion rules

For value conversion rules in JavaScript, please refer to Table 3-2. JavaScript type conversions in the book "JavaScript – The Definitive Guide". Some of the more noteworthy places are:

1. The result after undefined is converted to number is NaN.
2. The result of converting null to number is 0.
3. The result of converting the empty string "" into number is 0.
4. The result after converting -0 to string is "0".
5. The result of converting the empty array [] to number is 0.
6. When an array with only one number member (such as [9]) is converted into number, the result is the numer value (9).

When JavaScript converts string to number, there are two interesting rules:

1. JavaScript will delete the blank characters at the beginning and end of the string before converting it, so a string like "42" can be successfully converted into the number 42.

2. After removing the blank characters at the beginning and end, if the string still contains non-numeric characters, the string will be converted to NaN. For example: "3 m" will be converted to NaN.

Example:

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console.log(10 * " 3 ");//30
console.log(10 * "3 m");//NaN, "3 m" will be converted to NaN

Value type conversion and comparison

In JavaScript, the use of the equal operator (==) involves value type conversion: if the value types on both sides of the == operator are inconsistent, then JS will convert them into consistent types before making a judgment. It should be noted that two values ​​of different types may be equivalent after type conversion, but this does not mean that the result of using the == operator on them must be true. A simple example is undefined and false: after converting undefined to boolean type, the result happens to be false, but in fact the result of undefined==false is false.

Explicit type conversion

Using JavaScript's automatic type conversion is very convenient, but it can also easily cause problems such as code maintainability. In order to make the program code clearer and reduce ambiguity, explicit type conversion is sometimes used in JS programs:

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Number("3")//3
String(false)//"false"
Boolean([])//true

In most cases, the result of explicit type conversion is consistent with the result of JS automatic type conversion; but there is a special case: when null or undefined is automatically converted to Object, JS will throw a TypeError error; but when explicit type conversion When converting null or undefined to Object, JS will return an empty object:

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console.log(Object(undefined));//empty object
console.log(Object(null));//empty object

At the same time, if the type specified by explicit conversion is different from the type specified by JS automatic conversion, the results obtained will also be different. For example, the result of undefined==false mentioned before is false; if the conversion type is explicitly specified as Boolean, the result will be true:

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console.log(undefined == false);//false
console.log(Boolean(undefined) == Boolean(false));//true

Use of automatic type conversion

In JS, you can use automatic conversion of value types and use operators to achieve the same effect as explicit conversion, such as:

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console.log(false "");//"false"
console.log(false);//0
console.log(!!3);//true