The delete operator removes a property from an object. It cannot remove a variable. So the answer to the question depends on how the global variable or property is defined. (1) If it is created with var, it cannot be deleted. For example: (2) If it is created without var, it can be deleted. Technical Explanation 1. Using var In … Read more
You have to differentiate between cases: Variables can be undefined or undeclared. You’ll get an error if you access an undeclared variable in any context other than typeof. A variable that has been declared but not initialized is undefined. Undefined properties , like someExistingObj.someUndefProperty. An undefined property doesn’t yield an error and simply returns undefined, which, when converted to a boolean, evaluates to false. So, … Read more
You can just check if the variable has a truthy value or not. That means will evaluate to true if value is not: null undefined NaN empty string (“”) 0 false The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section. Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance … Read more
JQuery library was developed specifically to simplify and to unify certain JavaScript functionality. However if you need to check a variable against undefined value, there is no need to invent any special method, since JavaScript has a typeof operator, which is simple, fast and cross-platform: It returns a string indicating the type of the variable … Read more
you’re miss-reading this error. props is not undefined, what is calling props is undefined, which is the this keyword. you can manually set the context of the map function by passing in a second parameter this.props.buildings.map(this.renderBuildings, this) or bind it inline this.props.buildings.map(this.renderBuildings.bind(this))
You should quote your array keys: As is, it was looking for constants called department, name, email, message, etc. When it doesn’t find such a constant, PHP (bizarrely) interprets it as a string (‘department’, etc). Obviously, this can easily break if you do defined such a constant later (though it’s bad style to have lower-case … Read more
The typeof operator will check if the variable is really undefined. The typeof operator, unlike the other operators, doesn’t throw a ReferenceError exception when used with an undeclared variable. However, do note that typeof null will return “object”. We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead: For more … Read more
In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as: null is an assignment value. It can be assigned to a variable as a representation of no value: From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object. and
If you have any source file that includes GameObjects.h before ProjectilObject.h or does not include ProjectilObject.h directly, then the compiler will first find the declaration of ProjectilObject through the include in GameObjects.h before knowing what WorldObject is. That is because GameObjects.h first includes ProjectilObject.h and then declares WorldObject. In that case the include of GameObjects.h … Read more
You can safely use the typeof operator on undefined variables. If it has been assigned any value, including null, typeof will return something other than undefined. typeof always returns a string. Therefore