无法返回子字符串值angular typescript

let str = "Hello, World!";
let subStr = str.substring(0, 5); // 提取从索引0到索引4的子字符串
console.log(subStr); // 输出: "Hello"

let str = "Hello, World!";
let subStr = str.slice(0, 5); // 提取从索引0到索引4的子字符串
console.log(subStr); // 输出: "Hello"

let str = "Hello, World!";
if (str.length > 5) {
    let subStr = str.substring(0, 5);
    console.log(subStr); // 输出: "Hello"
} else {
    console.log("字符串长度不足");
}

function getSubstring(str: string, start: number, end: number): string {
    if (typeof str !== 'string') {
        throw new Error("参数类型错误");
    }
    return str.substring(start, end);
}

let result = getSubstring("Hello, World!", 0, 5);
console.log(result); // 输出: "Hello"