var functionName = function() {} vs function functionName() {}

pasta_paul

I've recently started maintaining someone else's JavaScript code. I'm fixing bugs, adding features and also trying to tidy up the code and make it more consistent. The previous developer used two ways of declaring functions and I can't work out if there is a reason behind it or not. The two ways are: var functionOne = function() { // Some code }; And, function functionTwo() { // Some code } What are the reasons for using these two different...

Originally asked by Richard Garside on Stack Exchange (stackoverflow) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

The difference is that functionOne is a function expression and so only defined when that line is reached, whereas functionTwo is a function declaration and is defined as soon as its surrounding function or script is executed (due to hoisting ). For example, a function expression: // TypeError: functionOne is not a function functionOne(); var functionOne = function() { console.log("Hello!"); }; And, a function declaration: // Outputs: "Hello!" functionTwo(); function functionTwo() { console.log("Hello!"); } Historically, function declarations defined within blocks were handled inconsistently between browsers. Strict mode (introduced in ES5) resolved this by scoping function declarations to...

Full answer by Greg: https://stackoverflow.stackexchange.com/a/336868 (CC BY-SA 4.0).

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