When performing +, -, *, /, ==, !=, etc. in JavaScript, if the value types on both 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 before performing operations. 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 numeric value, it will return NaN). For example:
The code copy is as follows:
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 the 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 is NaN after undefined conversion to number.
2. The result is 0 after converting null to number.
3. The result is 0 after converting the empty string "" to number.
4. The result is "0" after converting-0 to string.
5. The result is 0 after converting the empty array[] into number.
6. The array with only one number member (such as [9]) is converted into number and the result is the number value (9).
When JavaScript converts string to number, there are two more interesting rules:
1. JavaScript will delete the whitespace characters at the beginning and end of the string before converting it, so strings like "42" can be successfully converted to the number 42.
2. After deleting the whitespace 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:
The code copy is as follows:
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 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 judgments. Be careful that two different types of values may be equivalent after type conversion, but this does not mean that the result of using the == operator for them must be true. A simple example is undefined and false: the result of undefined after converting undefined to boolean type is exactly false, but in fact the result of undefined==false is false.
Explicit type conversion
Automatic type conversion using JavaScript is very convenient, but it can easily cause problems such as code maintainability. To make the program code clearer and less ambiguity, explicit type conversions are sometimes used in JS programs:
The code copy is as follows:
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 null or undefined is explicitly converted to Object, JS will return an empty object:
The code copy is as follows:
console.log(Object(undefined));//empty object
console.log(Object(null));//empty object
At the same time, if the type specified by the explicit conversion is different from the type specified by the automatic conversion of JS, the results obtained are also different. For example, the result of undefined==false mentioned earlier is false; if the conversion type is explicitly specified as Boolean, the result is true:
The code copy is as follows:
console.log(undefined == false);//false
console.log(Boolean(undefined) == Boolean(false));//true
Use of automatic type conversion
In JS, automatic conversion of value types can be used to achieve the same effect as explicit conversion through operators, such as:
The code copy is as follows:
console.log(false + "");//"false"
console.log(+false);//0
console.log(!!3);//true