How to have a statement identify an undefined variable?

 

This question already has an answer here:

  • How to check a not-defined variable in JavaScript 11 answers
  • Detecting an undefined object property 40 answers 
    • > exponent != 'undefined'

    You need to understand the Abstract Equality Comparison Algorithm. The above attempts to compare the value of exponent with the string "undefined". Since exponent is defined but has not been assigned a value, it will return the value undefined which is not equal to the string "undefined" (according to the algorithm above).

    So you can compare its value to the undefined value: 

    exponent != undefined

    or you can compare the type of the value with an appropriate string value using the typeof operator (since it always returns a string): 

    if (typeof exponent != 'undefined')

    Whether you use the strict or abstract versions above ( !== or != respectively) doesn't make any difference in this case.

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