# Stupid trick: Writing WebAssembly from Scratch I managed to [write WebAssembly opcodes from scratch](https://www.youtube.com/watch?v=pkw9USN_Tko), you can too! ### Step 1: let's learn LEB Lesson 1: -64 through 63 are just those number directly! That's plenty of numbers, we're done! ### Step 2: memorize the magic number! It's 0, a, s, m. Another stupid trick is memorizing ascii codes - remember, A is 65 (0x41) in ASCII, and lowercase starts 0x20 later at 97 (0x61) - 0, 97, 115, 109 - just remember those! ### Step 3: memorize the version number! It's 1, little endian: 0x01 0x00 0x00 0x00 ### Step 4: memorize the types section! typeSection = [0x01, 0x05, 0x01, 0x60, 0x00, 0x01, 0x7f] Function section, 5 more bytes, there is one type, it's a function (-0x20), 0 arguments, 1 return value, an int (-0x01)

    0x01, // Type section
    0x05, // this section has 5 more bytes
    0x01, // There is 1 type
    0x60, // it's a function
    0x00, // it takes zero arguments
    0x01, // it has one return value
    0x7f, // which is an int

### Step 5: the functions section 3, 2, 1, 0! (for each function, point to its type) ### Step 6: the exports section 7, 5, 1, name length, name, 0, 0

    0x07, // Export section
    0x05, // this section has 5 more bytes
    0x01, // there's one export
    0x01, // the name of the function has one byte
    'x'.codePointAt(0), // let's call our exported function "x"
    0x00, // it's a function
    0x00, // it's the function at index 0

### Step 7: memorize the skeleton of the code section

    0x0a, // Code section
    code.length + 2, // there are code.length + 2 more bytes in this section
    0x01, // there's one function in here
    code.length, // this function is code.length bytes long
    ...code

### Step 8: memorize a few operands 0x41 means constant i32, for some reason this is ascii A! i32.add 0x6a start at 106 i32.sub 0x6b i32.mul 0x6c i32.div_s 0x6d 0x0b // END opcode ]

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