What is the purpose of the var keyword and when should I use it (or omit it)?

> ***NOTE**: This question was asked from the viewpoint of ECMAScript version 3 or 5. The answers might become outdated with the introduction of new features in the release of ECMAScript 6.* What exactly is the function of the `var` keyword in JavaScript, and what is the difference between var someNumber = 2; var someFunction = function() { doSomething; } var someObject = { } var someObject.someProperty = 5; and someNumber = 2; someFunction = function() { doSomething; } someObject = { } someObject.someProperty = 5; ? When would you use either one, and why/what does it do?
When chaining var declarations, does putting a newline after a comma affects the behavior? var x=1, y=2, [return]z=3;

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