JavaScript Knowledge Points Summary (v) The difference between two equal numbers (= =) and three equal numbers (= =) in JavaScript _javascript tips

I. The role of JavaScript "= ="

1. When the content on either side of = = is a string, the contents of the string are compared to equal.
2. When the content on both sides of = = is a number, compare the size of the number is equal.
3. When the content on either side of the = = is the object or the function property of the object, compare the memory address for equality.

Two, = = and = = Difference

= = For general comparison, = = = for strict comparison, = = In the comparison can be converted data types, = = strictly compared, as long as the type does not match to return flase.

An example is provided:

The result of <script type= "Text/javascript" >
alert ("\" ==true) is: "+ (" "==true));
The result of alert ("\" \ "===true) is:" + ("" ===true);
The result of alert ("\" = = =: "+ (" = =));
The result of alert ("\" \ = = = =) is: "+ (" "= = =));

"1" = = True Type is different, "= =" will be the first type conversion, convert true to 1, that is, "1" = 1; At this time, the type is still different, continue to type conversion, "1" to 1, that is 1 = 1; At this point, "= =" Both sides of the type are numeric, relatively successful!

"1" = = True to the left of the character type, the right side is a bool Boolean, the left side of the different types, the result is false;

"1" = = 1 to the left of the character type, the right is an int numeric type, the left and the opposite types, the result is false;

Run Result:

Summarize:

= = and = = Difference: "= =" only required equal value; "= = =" Required value and type are equal

Here's a detailed description of the difference between three equals and two equals in JavaScript.

= = Equality equivalent, = = identity identity.

= =, when both sides of the value types are different, you have to do type conversion, and then compare.

= =, does not do the type conversion, the type difference must be unequal.

The following are described separately:

First say = = =, this is relatively simple.

The following rule is used to determine whether two values are equal to = =:

1, if the type is different, [not equal]

2, if two are numeric, and is the same value, then [equal]; Exception is, if at least one of them is Nan, then [not equal]. (To determine whether a value is Nan and can only be judged by isNaN ())

3, if two are strings, each position characters are the same, then [equal]; otherwise [unequal].

4, if two values are true, or all false, then [equality].

5, if two values all refer to the same object or function, then [equal]; otherwise [unequal].

6, if two values are null, or are undefined, then [equality].

again = =, according to the following rules:

1, if two value types are the same, do = = = comparison.

2, if two value types are different, they may be equal. Type conversions and comparisons are made according to the following rules:

A, if one is null, one is undefined, then [equals].

B, if one is a string, and one is a numeric value, the string is converted to a numeric value and then compared.

C, if either value is true, convert it to 1 and compare it to 0 if any value is false.

D, if one is an object, and the other is a numeric value or a string, the object is converted to the underlying type for a comparison. object to the underlying type, using its ToString or ValueOf method. JS core built-in class, will try to valueof before ToString, the exception is date,date use of ToString conversion. Non-JS core objects,

Order said (more trouble, I do not understand)

E, any other combination, [not equal].

Example:

"1" = = True
Type, true will first be converted to a value of 1, now become "1" = 1, and then convert "1" to 1, compared to 1 = 1, equal.

= Assignment operator
= = equals
= = = Strictly equal To

Cases:

var a = 3;
var B = "3";

A==b returns True
A===b returns False

Because the type of a,b is different.
= = = Used for strict comparative judgment