javascript エラー「Uncaught ReferenceError: xxx is not defined」の解決方法

const foo1 = 'msg';

console.log(foo)

Uncaught ReferenceError: foo is not defined

Uncaught ReferenceError: foo is not defined

let foo = "msg"

console.log(foo)

if (typeof foo !== 'undefined') console.log(foo)

function foo(){

    let str = "abc"

    console.log(str)

}

console.log(str)
// Uncaught ReferenceError: str is not defined

let str = "abc"

function foo(){    

    console.log(str)
}

console.log(str)
// abc

foo()
// abc