How to mock a function inside a function to be tested

there is now a file module.js
that contains two functions function a and function b
where b depends on a

function a () {
    // do something
}

function b () {
    // do something else
    var A = a()
    // Ba
}

how to use Jest to mock function a

when measuring b?
Mar.18,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
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MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b313ea-2bd62.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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