Summary of JavaScript logical operators

Preface

With regard to the logical operators in JavaScript, we often use them but may not know some of their mechanisms and usage.

Mechanism

First of all, we need to know that the priorities of several logical operators are different (for the complete operator priority, see operator priority), and logic is not > Logical and > Logical or > Conditional operator (3-item operator). Operational order conditional operators are right-to-left, while logical AND and logical OR are both left-to-right.

// The precedence of the expression causes different results 
false && true || true   //  The result is  true
false && (true || true)   //  The result is  false

The logical operation expression returns the value of the word expression instead of 1 Boolean It's just that many times where we use logical expressions helps us cast, such as if Statements, etc.

Logical operators are usually used for Boolean (logical) values. In this case, they return 1 Boolean value. However, & & The and operators return the value of 1 specified operand, so these operators are also used for non-Boolean values. At this time, they will also return a non-Boolean value.

Logical and expression1 && expression2 The mechanism of is that if expression1 Can be converted to true Then return expression2 Otherwise, return expression1 .

Logical or expression1 || expression2 The mechanism of is that if expression1 Can be transformed into true Then return expression1 Otherwise, return expression2 .

Logical non- !expression If expression Can be transformed into true Returns the false Otherwise, return true .

expression may be any 1 type, but not 1 is a Boolean value.

Will be converted to false The expressions of are:

null NaN 0 Empty string ("" or "" or "`" "`) undefined

What needs special attention is that undefined Some expressions return the undefined For example, there is no setting return The return value of the function execution of is undefined .

Although && And || Operators can use non-Boolean operands, but they can still be considered Boolean operators because their return values can always be converted to Boolean values. If you want to explicitly convert their return values (or expressions) to Boolean values, use double non-operators (that is, !! ) Or Boolean Constructor.

Double non-operator !! You can cast any value to a Boolean value, which is often used where conditional judgment is needed.

Short circuit calculation

The mechanism of logic operation also exists short circuit calculation:

(some falsy expression1) && (expression2) The result of short circuit calculation is false. (some truthy expression1) || (expression2) The result of short circuit calculation is true.

Short circuit means that in the above expression expression2 Part will not be executed, so expression2 Any side effects of will not take effect (for example, if expression Is a function call, this call will not occur). The reason for this phenomenon is that the value of the whole expression has been determined after the first operand has been evaluated.

Usage

Utilization expression12 Logical operators support arbitrary types and short-circuit calculation. We can apply logical operators to some special places.

Logical and

Logical and can be used to get the first false value, such as expr1 && expr2 && expr3 Is returned when there is a false value in it. It can also be analogized to execute the last expression when the previous expressions are all true to simplify the code of judgment logic, such as x > 0 && a() This can be used instead of if Statement.

Logical or

Logic may be used to set default values. For example, your function requires the user to enter 1 parameter, and if the user does not enter 1 default value. this.a = param || {} .

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