handlebars

Postconditions — what we check

• compile · invalid-template-syntax

  error
  WhenTemplate string contains syntax errors (e.g., unclosed mustache, invalid expression)
  ThrowsHandlebars.Exception (extends Error, with lineNumber and column properties)
  Required handlingCaller MUST wrap compile() in try-catch when compiling user-provided or dynamic templates.
  costmediumin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[1][2]
• template execution (TemplateDelegate) · template-execution-partial-not-found

  error
  WhenTemplate references a partial ({{> partialName}}) that is not registered via Handlebars.registerPartial() at execution time.
  ThrowsHandlebars.Exception with message "The partial <name> could not be found"
  Required handlingCaller MUST wrap the template execution call in try-catch when using partials, especially when partial names come from dynamic data or when partial registration may be incomplete. Alternatively, register all required partials before calling any template that references them.
  costmediumin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[3][4]
• template execution (TemplateDelegate) · template-execution-strict-undefined-variable

  warning
  WhenTemplate is compiled with strict: true option (or assumeObjects: true) and the template references a property that does not exist on the context object. Without strict mode, missing properties silently render as empty string.
  ThrowsHandlebars.Exception with message '"<varName>" not defined in <obj>'
  Required handlingCallers using strict mode MUST wrap template execution in try-catch, or validate that all template-referenced properties exist on the context object before rendering. Common pitfall: strict mode is enabled for security but error handling is not added.
  costlowin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[1][4]
• template execution (TemplateDelegate) · template-execution-prototype-access-silent-failure

  warning
  WhenTemplate accesses a prototype property (e.g., {{user.trim}}, {{arr.map}}) on an object. Since Handlebars 4.6.0, prototype access is forbidden by default and silently returns undefined rather than the property value. This is NOT an exception — it is a silent failure where data appears missing in the rendered output.
  ThrowsNo exception — silently returns undefined; template renders with empty value
  Required handlingCallers MUST either (a) use plain data objects without prototype methods in template context, (b) set allowProtoPropertiesByDefault: true or allowedProtoProperties in runtime options, or (c) restructure data to only pass own properties. Silent failure is the primary risk: rendered output appears partially blank without any error signal.
  costmediumin prodsilent failureusers seelost datavisibilitysilent
  Sources[5][6]
• precompile · precompile-invalid-template-syntax

  error
  WhenTemplate string contains syntax errors (unclosed mustache, invalid expression, unknown helper in knownHelpersOnly mode). Identical behavior to compile() at parse/compile time.
  ThrowsHandlebars.Exception (extends Error, with lineNumber and column properties)
  Required handlingBuild scripts and CLI tools calling precompile() MUST wrap in try-catch or the entire build step will crash. precompile() is often called without error handling because it runs in a build context where errors are "assumed to be caught by the build tool" — this is incorrect; Handlebars.Exception propagates unless caught.
  costlowin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[1][2]
• precompile · precompile-invalid-input-type

  error
  Whenprecompile() receives a non-string, non-AST input (e.g., null, undefined, number, Buffer). Unlike compile() which may coerce, precompile() enforces the input type strictly.
  ThrowsHandlebars.Exception with message "You must pass a string or Handlebars AST to Handlebars.precompile. You passed <type>"
  Required handlingValidate that precompile() input is a string before calling, especially when template content is loaded from files, databases, or APIs that may return null/undefined.
  costlowin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[2]
• template · template-version-mismatch

  error
  WhenThe precompiled template specification was generated by a different version of Handlebars than the current runtime. This happens when the handlebars version used at build time (precompile) differs from the version installed at runtime (template). Common in microservices where build and runtime environments diverge, or after upgrading handlebars without recompiling templates.
  ThrowsHandlebars.Exception: "Template was precompiled with an older/newer version of Handlebars than the current runtime. Please update your precompiler/runtime."
  Required handlingCallers MUST wrap Handlebars.template() in try-catch. Ensure precompiled templates are regenerated whenever handlebars is upgraded. Pin handlebars to exact versions in both build and runtime environments to prevent version drift.
  costhighin prodimmediate exceptionusers seeservice unavailablevisibilityvisible
  Sources[1][4]

Sources

Every postcondition cites at least one of these. Numbered to match the footnotes above.

1. [1]handlebarsjs.com/api-reference/compilation.htmlhttps://handlebarsjs.com/api-reference/compilation.html
2. [2]github.com/handlebars-lang/handlebars.jshttps://github.com/handlebars-lang/handlebars.js/blob/master/lib/handlebars/compiler/compiler.js
3. [3]handlebarsjs.com/api-reference/runtime-options.htmlhttps://handlebarsjs.com/api-reference/runtime-options.html
4. [4]github.com/handlebars-lang/handlebars.jshttps://github.com/handlebars-lang/handlebars.js/blob/master/lib/handlebars/runtime.js
5. [5]handlebarsjs.com/api-reference/runtime-options.htmlhttps://handlebarsjs.com/api-reference/runtime-options.html#options-to-control-prototype-access
6. [6]github.com/handlebars-lang/handlebars.jshttps://github.com/handlebars-lang/handlebars.js/blob/master/lib/handlebars/internal/proto-access.js