faint! What does the interview question console.log(([][[]] + [])[+!![]]+([]+{})[+!![]+ + !![]]) output?

This article brings you relevant knowledge about the front-end. It mainly shares with you an interview question about console.log. Friends who are interested can take a look at it below. I hope it will be helpful to everyone.

Preface

Interview questions console.log(([][[]] [])[ !![]] ([] {})[ !![] !![]]) output What? I encountered this interview question during the interview. The interview question examined type conversion, implicit type conversion, operators, etc. I couldn't answer it. It's not that I don't know how to answer it. It's just that I got dizzy just reading the question. However, it is indeed an interview question, so try to solve it.

Knowledge required to solve this problem

• " ": One yuan will perform the same conversion as using Number()
• If the number is the addition of two numbers, and any operand is an object, numerical value or Boolean, their toString method is called to convert it to a string
• "!": non-operator, this operation always returns a Boolean value
• "!!": forced conversion to Boolean value

Solve this problem ([][[]] [])[ !![]] ([] {}) [ !![] !![]]

First we have to clarify its execution order

First execute([][[]] []) , first execute the [][[]] inside. When calculating [][[]], it actually executes the key in the [] empty array. The value of []

1. First, the JavaScript engine parses the empty array[]. An empty array has no elements.
2. Next, the engine attempts to access the properties of the empty array using [] as the property name. In this case, since [] itself is an array, the JavaScript engine will try to convert it to a string. Since [] is an empty array, the result after conversion to a string is an empty string "".
3. Finally, the engine will try to access the property named "" in the empty array. Since this property does not exist in the empty array, the result is undefined.

So, the result of executing [][[]] is undefined.

Then ([][[]] [])The next execution is (undefined []) will perform implicit type conversion, and Convert undefined to the string "undefined", convert [] to "", and then splice the two strings together. Therefore, the result is "undefined"

Then execute [!![]], !![] empty array is converted to a Boolean value of true, and the unary operation will Convert to numeric value, true is to convert true into number 1. The result is [1]. So we can get ([][[]] [])[ !![]] is undefined[1]The result of this is the string 'n'

The next step is to execute ([] {})[ !![] !![]] and then add the result of this part Add the above result

1. First execute ([] {}), and calculate [] {}. This triggers an implicit type conversion, converting the empty object to the string "[object Object]", and then concatenating the two strings together. Therefore, the result is '[object Object]'.

2. Execute [!![] !![]] again, !![]The result obtained after parsing is 1, so after executing this part, we get is to add the value 1 to 1 1, and the result is [2].

3. Then the two parts combined are '[object Object]'[2], which is the third value of the string b

Finally, add the result 'n' obtained in the previous part and the result 'b' obtained in the latter part to get the final result 'nb'

Summary: If this problem cannot be solved for the time being, there may be several reasons. The first is that the execution sequence is not clear, and the second is that the execution sequence is not clear.[][[]]This is actually to get the value of the empty array with the key [], and because the key of the array is a string, [] will be converted to '' cannot be found, so undefined is returned. Do you know this interview question now?

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