map方法写在外面可以正常，写在function报错

Question

map方法写在外面可以正常，写在function报错

function num() {

                const c = this.data.map(item => {
                    return item * 2
                })
                console.log(c)
            }

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'map')
at num (index.html:31:47)
at Proxy.mounted (index.html:37:17)
at vue3.2.45.js:4279:44
at callWithErrorHandling (vue3.2.45.js:1744:26)
at callWithAsyncErrorHandling (vue3.2.45.js:1753:25)
at Array.hook.__weh.hook.__weh (vue3.2.45.js:4253:33)
at flushPostFlushCbs (vue3.2.45.js:1925:51)
at render (vue3.2.45.js:7752:13)
at mount (vue3.2.45.js:5983:29)
at Object.app.mount (vue3.2.45.js:10924:27)

Answer 1

你没理解什么是this
该传参传参，不要老想用全局变量