All of this works because TypeScript allows us to index any object as long as the index's type is a union of all the possible keys, so it knows that the key is valid. Object literal property names can be defined in two ways: using literals or using strings. I am sending an object with one key-value pair. Otherwise, it's still just a string. But I don't know which of the five will be sent. Expected behavior: typeName is T, but the type is incompatible with BVTemplate because the object literal declaration stores it as string.string is not compatible to T.. Actual behavior: I would hope this example could work. The test looks great and answers some of the questions I was going to ask next. Hopefully this doesn't happen in a nonadversarial environment (you should never add enumerable properties to Object.prototype), but it is another reason that for-in produces string keys even for object literals. The commented string works perfectly, and in JS land those two things should be equivalent. Here we made on into a generic method.. That’s for completions on object literal keys, not values, which could be anything. I think this looks good overall, just going to leave some suggestions for names and code style. Both the following are the same: obj = { thetop : 10 }; obj = { "thetop" : 10 }; In ES5 and earlier, you cannot use a variable as a property name inside an object literal… Suppose we need to add a function to the person object later this is the way you can do this. The code will create an object with a property named thetop that has a value of 10. The only option you had was to create the object literal, assign the variable property name with value and pass the resulting object to the animate method. How to use variable as an Object key in JavaScript javascript 1min read In this tutorial, we are going to learn about using the variable as a key in a JavaScript object literal with the help of … When a user calls with the string "firstNameChanged', TypeScript will try to infer the right type for K.To do that, it will match K against the content prior to "Changed" and infer the string "firstName".Once TypeScript figures that out, the on method can fetch the type of firstName on the original object, which is string in this case. ES6 defines ‘ComputedPropertyName’ as part of the grammar for object literals, which helps use a variable for a key. Enforces consistent object literal property quote style. TypeScript Type Template. Let’s say you created an object literal in JavaScript as − var person = { firstname:"Tom", lastname:"Hanks" }; In case you want to add some value to an object, JavaScript allows you to make the necessary modification. That includes the toString() and the hasOwnProperty() methods, for example. var obj = { [key]: value } Where key can be any sort of expression (e.g. For example, these two objects are equivalent: var object1 = { property: true }; var object2 = { “property”: true }; While object (lowercased) represents all non-primitive types, Object (uppercased) describes functionality that is common to all JavaScript objects. Lastly, we use a type guard here to say that, if this function returns true, any further usage of key will be of the specified type. Rule: object-literal-key-quotes. If you want to iterate over the keys and values in an object, use either a keyof declaration (let k: keyof T) or Object.entries. a variable) returning a value.. { thetop : 10 } is a valid object literal. With ECMAScript 2015 you are now able to do it directly in object declaration with the brackets notation: . 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