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How to Debug a Contract

Debugging a sCrypt contract is as easy as debugging TypeScript, since it is just TypeScript.

Use console.log()

You can use console.log() to print output to the console.

export class Demo extends SmartContract {

readonly x: bigint

readonly y: bigint

constructor(x: bigint, y: bigint) {
this.x = x
this.y = y

sum(a: bigint, b: bigint): bigint {
return a + b

public add(z: bigint) {
console.log(`z: ${z}`) // print the value of z
console.log(`sum: ${this.x + this.y}`) // print the value of this.x + this.y
assert(z == this.sum(this.x, this.y), 'incorrect sum')

Try it on Replit

After running the code, you should see the following output:

z: 3
sum: 3

Use Visual Studio Code debugger

You can use VS Code to debug sCrypt contracts, the same way as any other TypeScript programs. If you have created a project with the sCrypt CLI, you should have an auto-generated launch.json, containing everything needed for the debugger out of the box. To learn more about the VS Code TypeScript debugger, please refer to the official documentation.

You can set some breakpoints and choose Launch demo from the Run and Debug view (or press F5) to start the debugger instantly.


You need to change the contract file name in launch.json if needed.

Debug a test

If you want to debug a unit test written with the Mocha testing framework, choose Launch demo test from the Run and Debug view.


You need to change the contract test file name in launch.json if needed.