Javascript-替换字符串中的一定数量的字符并保留其余字符

Question

This is a weird question.

Essentially, I thought it would be a cool idea to let a user type a string, have that string converted to base64, and create an image.

Although I'm not learned in base64, I know images have header data, and need to be a certain length. I've managed to separate the base64 data that tells the colors of the image. Right now, I have a basic white image - you can check a js fiddle here - (not sure if it's working there, but it works in my browser).

Anyway, the variable "imgdata" is the raw data for my PNG image, and that's what I'd like to change. Unfortunately, that string seems to need a certain amount of characters, or the image won't work (guessing it's a size specified in the header?)

Because it is unlikely that a user inputted string will always reach that same number of characters when converted to base64, I would like to know how to replace the first part of a string, and leave the rest alone.

TL;DR

I have this string -

aaa

and I want to replace this string -

123456

Since string1 is only 3 characters, I only want it to replace the first 3 characters of string2 so the outcome would look like

aaa456

remember that string1 will vary in length.

Answer 1

Create a String containing the default values:

var fullLength = "abcdefg";

Get the user input as another string:

var user = "aaa";

Create a third string which is a combination of the user data and the end of the default:

var combined = user + fullLength.substring(user.length);

Because JavaScript is a weakly typed language, you need to ensure you are working with String objects. You can do this by concatenating with an empty string which will make sure all of the string functions work:

var userString = userInput + "";

Also, there are three very similar String functions, slice() , substr() , and substring() . Do some research on MDN to see which is best for you.

Answer 2

Javascript slice method can be used alone to solve the problem. The first parameter for slice defines the first character position to grab. The second parameter (stop position) can be omitted which means you will get the rest of the string. By using the length of a as first parameter means it will skip as many character as the length of a and then pad with the rest of the contents of b:

a + b.slice(a.length) // 'aaa456'