typeof foo和typeof（foo）/ delete bar和delete（bar）有什么区别，为什么我们都需要？

Question

As far as I can tell, typeof foo and typeof(foo) behave the same, and the latter is much more intuitive because I can just think of it as a function.

The same for delete() .

So, is there a difference? If not, why do we need both forms?

Answer 1

The parentheses are optional. typeof is an operator, not a function. That's like asking why do 5 + 3 and not 5 + (3) .

Answer 2

In most cases it will not make a difference however, it becomes useful when you are using typeof to evaluate the type of an expression.

typeof(1 + ' some text'); 

returns

string

However

typeof 1 + ' some text';

returns

"number some text"

Answer 3

Both typeof and delete are unary operators but with a big difference between the two: typeof evaluates its operand and delete does not. Operator precedence ( MDN reference ) is useful for discussion.

delete UnaryExpression

The delete operator does not evaluate its operand and ignores grouping operators '(' and ')' around it because they have higher precedence than delete . Hence delete could never be written as function call using the same syntax because a function call would evaluate function arguments. Delete can use property short cut or lookup syntax:

delete objectIdentifier.property name;  // is equivalent to
delete objectIdentifier["propertyName"]

In strict mode a syntax error is generated if you try using delete on anything that is not a reference to an object property that includes both the property name and object.

typeof UnaryExpession

The typeof operator evaluates its operand. If the operand expression is enclosed in parentheses, the entire expression between the parentheses is evaluated before applying typeof to the result. Using parentheses around an operator that can be identified using rules for operator precedence alone is redundant.

Summary

Using parentheses around an operand for delete doesn't help and is probably best avoided.

Use parentheses around a typeof operand to obtain the type of an expression result.

Placing parentheses around typeof and its operand, as in (typeof operand) can be useful in debugging a complex expression if operator precedence or automatic type conversions are not working as expected.

The use of parentheses around operands for delete and typeof does not create two kinds of syntax for the operators - the parentheses follow normal rules for parsing an expression.