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Console.log pitfall

Console live logging mishaps and pitfalls and how to avoid them

TipJavaScript July 16, 21
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According to MDN, there is one line in documenting console.log that sends a chill down your spine, and I could not find proper documentation for it.

Otherwise, many browsers provide a live view that constantly updates as values change.

So I wrote a simple test to see how that may happen.

var x = {name: "foo"};
console.log(x); = "bar";

x = {name: "oops"};

This will produce normal objects like this

Console.log output

But expand the objects:

Consolel log output

Notice how the first object name is no longer "foo", but rather "bar". The same happens when you are using console.dir and console.log({x}). The last log is because we reset the reference with a new object, which is not practical.

So to avoid that, you have two options. Either log out exact properties (, or as MDN suggests, clone the object before you log:

var x = {name: "foo"};
console.log(JSON.parse(JSON.stringify(x))); = "bar";

The output is

Console log output