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What is “undefined x 1” in JavaScript?
In Chrome 21, feeding [,] to the console outputs
[undefined x 1]
and feeding [undefined] outputs
[undefined]
What is the difference between [undefined] and [undefined x 1]?
What is the notation [undefined x 1]?
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The main difference between "undefined" and "not defined" is that "undefined" is a value that can be assigned to a variable, while "not defined" indicates that a variable does not exist.
It means null is equal to undefined but not identical. When we define a variable to undefined then we are trying to convey that the variable does not exist . When we define a variable to null then we are trying to convey that the variable is empty.
The undefined value is a primitive value used when a variable has not been assigned a value. The null value is a primitive value that represents the null, empty, or non-existent reference.
A variable that has not been assigned a value is of type undefined . A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned .
[,] is a sparse array. It has a length of 1, but no values (0 in [,] === false). It can also be written as new Array(1).
[undefined] is an array of length 1 with the value undefined at index 0.
When accessing the property "0", both will return undefined - the first because that property is not defined, the second because the value is "undefined". However, the arrays are different, and so is their output in the console.
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