Check out example codes for "vscode-jest intellisense not working". It will help you in understanding the concepts better.

Code Example 1

create jsconfig.json in the root directory of your project.
the file should contain the following:
{ "typeAcquisition": { "include": [ "jest" ] } }

after saving reload vscode.
Worked for me.

Learn ReactJs, React Native from akashmittal.com