Fixing the LangChain JS error: f-string Missing value for input ${node.name}

DanielJune 17, 2024June 17, 2024

I was working on a simple LangChain example in JavaScript and got the following error:

throw new Error(`(f-string) Missing value for input ${node.name}`)

The code was simple. I was testing a chain made of a prompt, a model, and a string output parser:

const prompt = PromptTemplate.fromTemplate(
    `Which country has the highest population in { continent } ?`
)

const chain = prompt
 .pipe(model)
 .pipe(new StringOutputParser())
const response = await chain.invoke({continent: 'Africa'})
console.log(response)

No matter what I was passing to the continent parameter I ended up with the same (f-string) Missing value for input ${node.name} error.

After almost two hours of agonizing trial and error, it seemed that the reason for the error was the extra white spaces from the curly braces surrounding the prompt input:

Just removing those spaces seemed to fix the error:

// { param } VS {param}

// ⛔️ throws an error because of the extra spaces in { continent } 
PromptTemplate.fromTemplate(
    `Which country has the highest population in { continent } ?`
)
// ✅ removing the extra spaces clears the error 
PromptTemplate.fromTemplate(
    `Which country has the highest population in {continent} ?`
)

Hope this one will help you save some time and nerves.