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Do you know about TypeOf?
I played with my colleagues yesterday. My colleagues gave me a code question about typeof, which is interesting. I would like to share it with you. By the way, I will consolidate my summary of typeof knowledge points. If any, point out and make progress together.
The Code is as follows:
<!DOCTYPE html>So what will the chrome debugger print when you run the code?
Please see,
Why does this result appear?
First, two null values in the analysis
Because Javascript will pre-Parse (hoisting) before executing the code ).
What is pre-resolution (hoisting?
In JavaScript, you can declare multiple var statements at any position of the function. They play the same role as the Declaration at the top of the function, the variable declared by var is assigned an undefined initial value. This behavior is called pre-resolution (hoisting ).
Therefore, before executing the above Code, Javascript will first pre-Parse all var variables (a, B, c) (hoisting). When a | null is executed, a is undefined, and because yes | is an operation, the result is null.
C | null is a truth.
Let's take a look at why typeof f is undefined? F is not defined. Shouldn't I report an error?
Yes. If we use an undefined variable, the browser reports an error.
However, when typeof is used to determine an undefined variable, undefined is returned.
What is the mechanism of typeof to determine the value type? Summary:
1. For numeric values, typeof Returns number. Note: NaN is also a numeric value because it represents a special non-numeric value in JavaScript.
2. For string type values, typeof returns string.
3. For boolean values, typeof returns boolean.
4. For objects, arrays, and null, typeof returns the object
5. For function types, typeof returns the function
6. If the number of operations is not defined (does not exist or is not assigned a value), undefined is returned.
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