This means that the code below will result in you having an array with duplicate elements.Ĭonst args = Array. Unlike object spread, where you won’t have duplicate properties because that’s just how JavaScript objects work (you can’t have an object with two fullName properties), with arrays you may end up with duplicate values, if you plan to implement something similar to our object example. Which object depends on how this is being invoked (used or called). Except that instead of spreading over keys and values, the operator spreads indexes and values. In JavaScript, the this keyword refers to an object. In this case, since we specified the fullName and website properties after the spread operator kicked in, the JavaScript engine is smart enough to know that we want to overwrite the original values of those properties that are coming from the original object. Think of this spread thing as extracting all the individual properties one by one and transferring them to the new object. It copies the properties of the adrian object, over to the newly created object, and then explicitly overwrites firstName and webSite. The this value is different in each context.What the code above does, is to spread over the adrian object and get all its properties, then overwrite the existing properties with the ones we’re passing. Static methods, static field initializers, and static initialization blocks belong to the static context. Constructors, methods, and instance field initializers ( public or private) belong to the instance context. This is the case for iterative array methods, the Promise() constructor, etc.Ī class can be split into two contexts: static and instance. But it is critically important for writing more advanced JavaScript code. Callbacks are typically called with a this value of undefined (calling it directly without attaching it to any object), which means if the function is non–strict, the value of this is the global object ( globalThis). JavaScripts this keyword is one of the hardest aspects of the language to grasp. When a function is passed as a callback, the value of this depends on how the callback is called, which is determined by the implementor of the API. When using these methods, the this substitution rules above still apply if the function is non-strict. Using (), you can create a new function with a specific value of this that doesn't change regardless of how the function is called. You can also explicitly set the value of this using the (), (), or Reflect.apply() methods. In typical function calls, this is implicitly passed like a parameter through the function's prefix (the part before the dot). Unicode character class escape: \p // Only for demonstration - you should not mutate built-in prototypes Number.Character class escape: \d, \D, \w, \W, \s, \S.Enumerability and ownership of properties.
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