A Brief Note for JavaScript Garden

Originally posted on 2015–10–31 on http://steveltn.me.

Recently I read about JavaScript Garden, and took some (extremely brief) notes.

Objects

• Everything is an object, except for null and undefined.
• Object.delete deletes a key.
• { k: v } and { 'k': v } are equivalent.

Prototypes

• Instances do not have prototypes by default, their constructors do.
• Prototype should be an instance, instead of constructors.
• in and for in both traverse the prototype chain.

this

• There are five scenarios.

Constructor

• this refers to the new object.

Scope

• There is no block scope
• foo = 42 defines global variable; var foo = 42 defines scope variable.
• Only function arguments and variables defined by var are scope variables.
• var and function statement will be moved to the top of their enclosing scope.

Arrays

• Don’t use for in because it traverses the prototype chain. Use for instead.
• You can truncate arrays by assigning length property.
• new Array() is broken. Use [1, 2, 3] instead.

Types

• == is broken, it coerces types. === doesn't.
• == and === compares identity instead of equality, when either side of it is an object.
• typeof and instanceof are broken.
• The correct way of getting an object’s class is by using Object.prototype.toString.
• The only use of typeof is to check if a variable is defined.
• The only use of instanceof is to deal with custom made objects that originate from the same JavaScript context
• JavaScript will apply type coercion wherever possible.

Core

• The scope of eval is broken.
• setTimeout and setInterval calls eval internally, if the arguments are strings.
• You can override the global variable undefined.
• JavaScript could guess wrong when automatically inserting semicolons.
• delete doesn't work when DontDelete attribute is set, hence can only be used to delete properties explicitly set on normal objects.

setTimeout and setInterval

• The timeout and interval are inaccurate.
• this in the callback refers to the global object
• setInterval can be blocked, causing function calls stacking. You can use setTimeout and set new timeouts in the callback instead.