Prevent error "is undefined" in if statement

 

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  • Detecting an undefined object property 40 answers

    • && will return true only if both the values of ( a && b ) a and b are truthy values.

    If first operand( expression ) is evaluated as false , second operand( expression ) is not evaluated at all because the result would always be false

    is better practice to use typeof to evaluate undefined variables: 

    if ( typeof array[index] !== 'undefined' && array[index].id == 1)
{
   // code
}

    Remember to check the string 'undefined' not the primitive value.

    More info in MDN

    No, if array is undefined, you will need an if statement that looks something like this: 

    if (array && array[index] !== undefined && array[index].id === 1) {
   // do things
}

    The first condition that if false will stop the evaluations of all the other conditions. The only way this will fail is if you are running you code in strict mode and have never declared var array

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