How JavaScript detects attributes in an object

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Detect attributes in an object

This tip is useful when you need to detect the existence of some properties and avoid running undefined functions or properties. If you plan to order some cross-compatible browser code, you may also use this trick. For example, if you want to use document.querySelector () to select an id and make it compatible with IE6 browsers, but this function does not exist in IE6 browsers, it is very useful to use this operator to detect the existence of this function, as in the following example:

If ('querySelector' in document) {document.querySelector ("# id");} else {document.getElementById ("id");}

In this example, if the querySelector function does not exist in document, then docuemnt.getElementById ("id") is called.

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