TypeScript Ambients Declaration
Ambient declarations files are saved with the extension (d.ts). A file with extension .d.ts must have the declare keyword prefixed to each root level definition. It makes clear to the author that there will be no code emitted by TypeScript. The author needs to ensure that the declared item will exist at runtime.
Ambient declarations tell the compiler about the actual source code exist elsewhere. If these source codes do not exist at runtime and we try to use them, then it will break without warning.
Ambient declarations files are like docs file. If the source changes, the docs need to be kept updated also. If the ambient declaration file is not updated, then we will get compiler errors.
The ambient variables and methods can be declared by using the declare keyword. The syntax for the ambient declaration is given below.
Syntax to access Ambient files:
As this is a JS file and we will not have time to re-write this library to typescript. But still need to use the doSum() function with type safety, then we can do this by using ambient declaration. Let us create an ambient declaration file.
Now, include this ambient declaration file (CalcSum.d.ts) into our TypeScript file.
Compile and executed the Main.ts file by using the following command on the console:
We will get the following output.