How do I check if an element is undefined?

2018-06-07 09:23:06

This question already has an answer here:

Detecting an undefined object property 40 answers

How to determine if variable is 'undefined' or 'null'? 24 answers

Using typeof :

if (typeof marcamillion == 'undefined') {
    console.log("Marcamillion is an undefined variable.");
}

Edit for the second question:

if ($(this).attr('value')) {
    // code
}
else {
    console.log('nope.')
}

if (typeof marcamillion === 'undefined') {
    console.log("Marcamillion is an undefined variable.");
}

请注意，使用===而不是==被认为是更好的风格。

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